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Page 6 of 26 Wang et al. Soft Sci 2023;3:34 https://dx.doi.org/10.20517/ss.2023.25

Furthermore, Xiong et al. developed one-step direct dilution to achieve stable, simple, and fast integration
[36]
of PEDOT:PSS film preparation and solvent treatment . Due to the PSS partly dissolved in the organic
solvent, the dilution process caused by PSS initially scattered away from the PEDOT-rich nanoparticle
aggregation significantly. The PEDOT:PSS film on PVDF was thin and flexible, so researchers could cut it
into any desired form and paste it onto a transparent PET substratum. This approach is expected to affect
the electrical and TE properties positively because it avoids the waste of pristine PEDOT:PSS, the limitation
of solvent types, the use of sophisticated instruments, and complex processes. The obtained PEDOT:PSS
thin films have high electrical conductivity of about 1,500 S/cm and a considerable ZT value of 0.1 with
facile treatment for the high or low-boiling-point solvents.

Previous works mainly used commercial PEDOT:PSS to prepare free-standing PEDOT-based films.
However, compared with the EDOT monomer, the cost of commercial PEDOT:PSS is too high, which is
unsuitable for application in the TE field. Song et al. filtered and obtained a flexible, independent PEDOT
[37]
nanofiber film through a simple self-assembled micellar soft template method [Figure 4A-D] . The
processing method perfectly solves the problem that PEDOT is difficult to handle due to its insoluble and
refractory properties. By incorporating SWCNTs into PEDOT nanofibers, the PEDOT nanofiber/SWCNTs
showed a power factor of 14.4 μW/mK . Ni et al. successfully used a modified self-assembled micellar
2
soft-template method and vacuum-assisted filtration to prepare flexible free-standing and high-conductive
[38]
(1,340 S/cm) PEDOT nanowire (NW) film . The film was treated with acid and base to further optimize
2
the TE properties. The power factor of the best PEDOT NW film was 46.51 μW/mK . Finally, a flexible TEG
assembled on polyimide consisting of six optimum PEDOT NW thin-film-silver thermocouples was
obtained to demonstrate TE power generation [Figure 4E and F].

PEDOT/inorganic composite TE materials
Despite the high electrical conductivity of PEDOT, its low Seebeck coefficient, typically ranging from 15 to
20 µV/K [10,39,40] , has become the main obstructive factor for its TE performance. Incorporating inorganic
semiconductors with a high Seebeck coefficient or power factor into PEDOT seems to be a good strategy,
which can fully use the advantages of PEDOT and inorganic components. Furthermore, PEDOT-based
composite films with high quality can be obtained by dispersing inorganic materials into PEDOT:PSS
solution, followed by a simple and effective vacuum filtration method.

Inorganic semiconductor materials, such as Te-, CuSe-, Bi-Te-based alloys, etc., are usually the first choice
to construct PEDOT-based composites. Song et al. have conducted excellent work in this field. In 2017, they
prepared PEDOT:PSS-functionalized Te (PF-Te) nanorods. The PEDOT:PSS layer provided excellent
dispersibility in the PEDOT:PSS solution, and then PEDOT:PSS/PF-Te composite films were obtained
[41]
through a vacuum filtration method using a 0.22 µm PVDF porous membrane [Figure 5] . Because of the
high Seebeck coefficient of Te nanorods and an energy filtering effect between the PEDOT:PSS layer and Te,
the Seebeck coefficient of the composites was enhanced from 15.6 to 51.6 µV/K. The highest power factor
reached 51.4 µW/mK for the sample containing 70 wt% PF-Te nanorods. Also, flexible PEDOT:PSS/Ag Te
2
2
nanocomposite films on the PES substrate were prepared through vacuum filtration and cold pressing
[42]
processes using PF-Te nanorods as the templates . An optimal PEDOT:PSS/Ag Te nanocomposite film
2
exhibited a power factor of 143.3 μW/mK at room temperature. In 2019, they prepared PEDOT:PSS-coated
2
Cu Se (PC-Cu Se ) NWs through a wet-chemical method, and then they fabricated PEDOT:PSS/Cu Se
y
x
y
x
y
x
nanocomposite films on flexible nylon membranes by vacuum filtration and cold-press processes
[Figure 6] . When the Cu/Se nominal molar ratio is three, the composite showed the highest power factor
[43]
of ~270.3 μW/mK at 300 K. Additionally, due to the nanoporous structure of a composite film, the intrinsic
2
flexibility of nylon and the good combination between nylon and film, the composite film showed excellent
flexibility. In another study, hot pressing at 200 °C and 1 MPa was employed to replace cold pressing to